For a few years the manufacturers were able to improve IQ at a per-pixel level at the same time as increasing the absolute number of pixels on each new generation of sensors.

For the APS-C that no longer seems to be the case (at least for Canon). However my take is that the cameras are continuing to improve as more features are added. Increasing pixel counts are no longer improving results at the per-pixel level, indeed they may be worse. But the IQ when measured at the print level is still improving slightly and of course the pixel count is still going up, and with good light those extra pixels come in handy for larger prints.

If I were you I'd hang on for a couple of weeks until DXOMark release results for the 500D and D5000. But assuming your basic premise that they are cheaper versions of the 50D and D90 you might find the comparison between those two cameras' sensors instructive: